Telephone set



Y 1943- F. FAIRLEY 2,318,681

- TELEPHONE SET Filed March 14, 1941 2 Sheets-Sheet 1 I INVENTOR E FA IRLEY M 11, 1943. F. FAIRLEY 2,318,681

TELEPHONE SET Filed March 14, 1941 2 Sheets-Sheet 2 I lNVE/VTOR BVF. FA/RLEV ATTOR EV Patented May 11, 1943 TELEPHONE SET Frank Fairley, London, England, assignor to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York Application March- 14, 1941, Serial No. 383,401 In Great Britain April 5, 1940 8' Claims.

This invention relates to mechanical amplifiers for use in conjunction with telephone or like systems and more particularly as receiving amplifiers in telephone subsets. The purpose of the amplifier is to allow the subset to be used over lines of greater attenuation than would otherwise be possible. The invention is thought to have a special value for use in a military field telephone network when transmission conditions are bad. It may have further application for point to point transmission over private telephone lines and in telephone subscribers sets for deaf persons.

The amplifier proposed is of the magnetic receiver-carbon transmitter type. It consists of a receiver mechanically coupled to a carbon microphone. It is considered undesirable that the internal connections or mechanical construction of the local battery subset should be disturbed when the amplifier is added. The amplifier may thus be easily applied to standard subsets in general use Without alteration.

One feature of the invention consists in a telephone subset characterized by an amplifier comprising a magnetic receiver mechanically coupled to a carbon-granule transmitter.

Another feature of the invention consists in an amplifier comprising a magnetic receiver mechanically coupled to a carbon-granule transmitter and arranged to be plugged in between a telephone subset and the cord of the telephone receiver and transmitter.

Yet another feature of the invention consists in an amplifier comprising a magnetic receiver, an input transformer over which the amplifier receiver is connected to terminals adapted to be connected to the receiver terminals of a telephone subset, a carbon-granule transmitter mechanically coupled to the amplifier receiver, and an output transformer over which the amplifier transmitter is connected to terminals adapted to be connected to the telephone receiver.

When used for military purposes, the amplifier may be situated where clamp is prevalent and may need to be protected. The carbon microphone transmitter requires to breathe, i. e. the microphone must be open to the atmosphere to allow for the mechanical movement of expansion and contraction within the carbon chamber; it is obviously therefore not possible to seal such a transmitter in bitumen or other protective compounds commonly used for protection against damp. It is proposed, however, to mount the microphone inside a copper box or other container provided with seals such as copper glass seals in such a way that it i hermetically sealed from the atmosphere but contains a space inside which may be filled with dry air or other gas. If space in the interior is suitably proportioned, the microphone may breathe without appreciably affecting its characteristic, compared with that in free air.

The interior space may be filled with an inert gas, which has the advantage that no burning of the particles of carbon will occur, and the length of life of the transmitter will be increased. Hydrogen is especially suitable for this purpose since it is capable of being absorbed by carbon. The pressure of gas can be predetermined with a view to obtaining the best results.

In order that the invention may be clearly understood, a description will now be given of certain of its embodiments, reference being made for this purpose to the accompanying drawings, in which:

Fig. 1 shows the circuit of a local battery sub set to which the invention can be applied;

Fig. 2 shows the same subset having a receiving amplifier interposed between the subset proper and the microtelephone, the amplifier having its own battery; and

v Fig. 3 shows the subset with a receiving amplifier interposed which makes use of the subset battery.

The essential parts of the subset circuit, omitting the arrangements for magneto ringing, are shown in Fig. l. The transmitter T is connected in series with the local battery B and with the primary of the induction coil I via the switchhook S. The secondary of the inductioncoil I, which is divided into two parts, is connected to the line L1L2 and the anti-sidetone network N. The receiver R is connected between the line and network in a conventional anti-sidetone circuit. The microtelephone comprising a transmitter T and receiver R are connected to the subset by means of a four-conductor cord C which plugs into a jack in the subset. The only part of the subset available for the insertion of a receiving amplifier without disturbing the interior of the subset is thus the cord C.

Fig. 2 shows the addition of a subset amplifier AMP of the type described above. The receiver AR of the amplifier is connected to the receiver terminals (or jack contacts) of the subset via the impedance matching transformer T1 and the transmitter AT of the amplifier is connected in series with the rheostat P which may be used for gain control and switch S2, a battery B2 and the primary of transformer T2. The secondary of transformer T2, which transformer is used for impedance matching, is connected to the subset receiver R. Switch S2 may be controlled independently but is preferably mechanically coupled to the subset switchhook S s that replacement of the subset transmitter-receiver on the switchhook breaks the battery circuit B2, thus avoiding excessive drain on this battery.

The disadvantages of the arrangement of Fig. 2 are, that the sidetone of the subset is increased by the amount of the gain of the amplifier AR, AT, that a separate battery B2 is required, and that mechanical arrangements for operating the switch S2 from the switchhook S are likely to be clumsy.

Fig. 3 shows an arrangement which overcomes these difiiculties. Switches S2 and battery B2 are no longer required, the subset battery B serving for both transmitter T and amplifier transmitter AT. It will be noted that there are 1 now two coupling paths for sidetone from transmitter T, one via the normal anti-sidetone circuit, i. e. induction coil I, T1 and AR, AT to T2 and R, and one via the common connections of AT with the transmitter T to T2 and R. If transformer T1 is connected in one sense the arrangement will, in general, sing but if transformer T1 is connected in the opposite sense, currents produced via the two sidetone paths tend to annul one another in transformer T2 and to reduce the sidetone below the value which it has with the arrangement of Fig. 2. Although the annulment of sidetone cannot be made perfect, clue to the complicated phase changes suffered by the sidetone currents at various frequencies in the two paths and due to the fact that the sidetone in the first path is dependent on the balance of thenetwork N against the line, by a suitable choice of circuit impedances and elements for transmitter T, amplifier transmitter AT and transformer T2, a material improvement of sidetone over that of Fig; 2 is possible.

Various other forms of coupling between AT, T2 and the transmitter circuit are possible, dependent on the circuit impedances, but the one shown in Fig. 3 has been found to be the best with the particular amplifier and subset which were used.

The arrangement proposed is not susceptible to singing if the impedance of battery B becomes high, as would be the case if the circuit AT, P, S2 and T2 were connected across the battery B, since the high impedance of the battery does not increase the coupling between the transmitter and receiver circuits but reduces it. The arrangement may be adapted to nonanti-sidetone subscriber sets or to other forms of subscribers sets such as central battery sets.

What is claimed is:

1. An amplifier unit comprising a container, an electromechanical amplifier comprising a receiver element and a microphone element, a first and. a second transformer, a primary winding of one transformer being connected to terminals on the container and a secondary winding of said one transformer being connected to said receiver element, a winding of the second transformer being connected to terminals on said container, and another winding of said second transformer being connected to a series circuit including said microphone element, means for adjusting current flow through said microphone element, two

pairs of additional terminals on said container and direct circuit connections between said pairs of additional terminals.

2. A telephone set circuit comprising line terminals, a multiwinding transformer, a resistance, said resistance and two of the windings of said transformer being connected in series between said line terminals, a source of current, a telephone transmitter, circuit-controlling contacts, means connecting said source, transmitter, contacts and a third winding of said transformer, a second transformer, means connecting one winding of said second transformer between one line terminal and a point common to said series-connected windings, an electromechanical amplifier comprising a receiver element and a microphone element, means connecting said receiver element across a second winding or said second transformer, a third transformer, a variable resistance, means connecting said microphone element, a winding of said third transformer and said variable resistance in a series circuit in shunt with said transmitter, a telephone receiver, and means connecting said receiver across a second winding of said third transformer.

3. A telephone set circuit comprising line terminals, a multiwinding transformer, a resistance, said resistance and two of the transformer windings being connected in series between said line terminals, a source of current, a telephone transmitter, circuit-controlling contacts, means con-v necting said source, transmitter, contacts and a third winding of said transformer, a second transformer, means connecting one winding of said second transformer between one line terminal and a point common to said series-connected windings, an electromechanical amplifier comprising a receiver element and a microphone element, means connecting said receiver element with a second winding of said second transformer, a third transformer, a variable resistance, a second source of current, additional circuitcontrolling contacts, means connecting in series said microphone element, said second current source, said additional contacts and a winding of said third transformer, a telephone receiver, and means connecting said receiver with a second winding of said third transformer.

4. A telephone set circuit as claimed in claim 2 in which the circuit connects between said telephone receiver and the second Winding of the third transformer, between the telephone transsaid transformer, circuit connections between the other of said pairs of terminals and one line terminal and a point common to two other windings of said transformer, said latter windings being connected in series across said line terminals; a second assembly unit comprising a receiver and microphone mechanically coupled so that energization of said receiver with voice currents actuate said microphone, a second and a third transformer, a variable resistance and a plurality of pairs of terminal members, one winding of said second transformer being connected across one pair of said members and a second winding of the second transformer being connected with said receiver, one winding of said third transformer being connected in series with said microphone and said resistance between a second pair of said members and a second winding of the third transformer being connected across a third pair of said terminal members, said second pair and a fourth pair of said terminal members being connected directly together; and a third assembly unit comprising a handset and a cord, said handset comprising a receiver and a transmitter and said cord including a separate pair of conductors for each of said handset receiver and transmitter and terminating in two pairs of terminal members, the handset receiver terminal members being adapted to interengage with said third pair of terminal members and the transmitter terminal members being adapted to interengage with said fourth pair of said terminal members, said first pair and said second pair of terminal members being adapted to interengage respectively with said other and said one pair of terminals of said first-named assembly unit, said units detachably interengaging through said terminals and terminal members whereby a complete telephone set maybe comprised solely of said first-named and said third-named assembly unit or of the three-named units.

6. In combination, an assembly unit comprising a multiwinding transformer, a source of current, line terminals and a plurality of terminal members, said transformer windings and said source of current being terminated at said line terminals and said terminal members; a telephone set comprising a transmitter and a receiver, and including a cord terminating in a plurality of terminal members for interengagment with the terminal members of the assembly unit; and a second assembly unit comprising a speech currents amplifier and a plurality of terminal members for interengagement with the terminal members of said first assembly unit and said telephone set, whereby the amplifier and the transmitter are fed with current from the source of current in said first assembly unit.

7. In combination, an assembly unit comprising a multiwinding transformer, a source of current, line terminals and two pairs of terminal members, said transformer windings and said source of current being terminated at said line terminals and said terminal members; a telephone set comprising a transmitter and a receiver, and including a cord terminating in a plurality of terminal members for interengagement with the terminal members of the assembly unit; and a second assembly unit comprising a speech currents amplifier including a machanically-coupled receiver element and microphone element, and a plurality of terminal members for interengagement with the terminal members of said first assembly unit and said telephone set, whereby the amplifier-microphone element and the transmitter are fed with current from the source of current in said first assembly unit.

8. A telephone set comprising line terminals, a speech current receiving circuit including a receiver and a speech current transmitting circuit including a transmitter connected across said terminals, said receiver and said transmitter being included in a handset and connected with the rest of their respective circuits through a multiconductor cord and a plurality of interengaging contact members, and an amplifier unit for connection between said handset and the rest of said respective circuits for inclusion in the receiving circuit to amplify the speech currents that may be incoming to the receiver in the receiving circuit, said unit including individual contact members for interengagement with said first-mentioned contact members.

FRANK FAIRLEY. 

